rust: internalize srcPathFromModuleSrcs

This was frequently misused (for example, in the prebuilts module, it
was used as a complex "assert(len(srcs))==1"), and can be superceded by
getCrateRoot anywhere it was used. It's now only called from
compiler.go, and can drop the second return parameter, as it was only
actually used by the prebuilt assert misuse.

Bug: 309943184
Test: m nothing
Change-Id: I6c92580bc8f0ecb7586c544056b5409e6dd280e7
diff --git a/rust/compiler.go b/rust/compiler.go
index 98bbcb6..899502a 100644
--- a/rust/compiler.go
+++ b/rust/compiler.go
@@ -541,7 +541,7 @@
 
 func (compiler *baseCompiler) crateRootPath(ctx ModuleContext) android.Path {
 	if compiler.Properties.Crate_root == nil {
-		path, _ := srcPathFromModuleSrcs(ctx, compiler.Properties.Srcs)
+		path := srcPathFromModuleSrcs(ctx, compiler.Properties.Srcs)
 		return path
 	} else {
 		return android.PathForModuleSrc(ctx, *compiler.Properties.Crate_root)
@@ -549,7 +549,7 @@
 }
 
 // Returns the Path for the main source file along with Paths for generated source files from modules listed in srcs.
-func srcPathFromModuleSrcs(ctx ModuleContext, srcs []string) (android.Path, android.Paths) {
+func srcPathFromModuleSrcs(ctx ModuleContext, srcs []string) android.Path {
 	if len(srcs) == 0 {
 		ctx.PropertyErrorf("srcs", "srcs must not be empty")
 	}
@@ -580,5 +580,5 @@
 	// TODO: b/297264540 - once all modules are sandboxed, we need to select the proper
 	// entry point file from Srcs rather than taking the first one
 	paths := android.PathsForModuleSrc(ctx, srcs)
-	return paths[srcIndex], paths[1:]
+	return paths[srcIndex]
 }